Deputies first responded to several calls reporting a person -- described as a black man wearing a checkered shirt -- was seen with a gun near Magnolia and Alpine avenues, Lt. Eddie Hernandez with the sheriff's Homicide Bureau said.
Additional callers told officials the man was moving eastbound on Magnolia, toward Long Beach Boulevard, a block away from the initial report.
At one point, Robertson was said to be actively shooting, according to Hernandez. It was not known where he was allegedly shooting, or if one or more witnesses reported this detail.
Responding deputies gave him "several commands" to drop the gun.
"We have witnesses that say that the suspect turned, pointed the gun at the deputies, and a deputy-involved shooting occurred," Hernandez said.
Investigators had yet to speak to the deputies involved as of Saturday afternoon, he said.
A person who wished to remain anonymous sent KTLA a 29-second video Saturday afternoon that apparently captured some of fatal incident.
In it, a sheriff's deputy follows a man who appears to be walking away from the official and next to an Arco gas station.
At least a dozen gunshots can then be heard being fired, and the man falls to the ground.
Once on the ground, the camera panned to focus solely on the man, who crawls next to the gas station and away from the gunfire.
After a pause in audible gunfire, at least a dozen additional shots are heard.
The camera then panned back to two deputies, who both had arms raised and were pointing guns in the man's direction.
At least one of the deputies then fired additional shots before the video ends.
It was not clear from the video if both deputies had fired their weapon, but when slowed down, more than 30 shots can be heard.
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